Elsevier

Cortex

Volume 111, February 2019, Pages 148-163
Cortex

Review
The frontal aslant tract (FAT) and its role in speech, language and executive function

https://doi.org/10.1016/j.cortex.2018.10.015Get rights and content

Abstract

In this review, we examine the structural connectivity of a recently-identified fiber pathway, the frontal aslant tract (FAT), and explore its function. We first review structural connectivity studies using tract-tracing methods in non-human primates, and diffusion-weighted imaging and electrostimulation in humans. These studies suggest a monosynaptic connection exists between the lateral inferior frontal gyrus and the pre-supplementary and supplementary motor areas of the medial superior frontal gyrus. This connection is termed the FAT. We then review research on the left FAT's putative role in supporting speech and language function, with particular focus on speech initiation, stuttering and verbal fluency. Next, we review research on the right FAT's putative role supporting executive function, namely inhibitory control and conflict monitoring for action. We summarize the extant body of empirical work by suggesting that the FAT plays a domain general role in the planning, timing, and coordination of sequential motor movements through the resolution of competition among potential motor plans. However, we also propose some domain specialization across the hemispheres. On the left hemisphere, the circuit is proposed to be specialized for speech actions. On the right hemisphere, the circuit is proposed to be specialized for general action control of the organism, especially in the visuo-spatial domain. We close the review with a discussion of the clinical significance of the FAT, and suggestions for further research on the pathway.

Section snippets

Anatomy and connectivity of the frontal aslant tract

Although its description in humans is relatively recent, connections between the lateral inferior frontal cortex and medial superior frontal cortex have been described previously in non-human primates. For example, Thiebaut de Schotten and colleagues (Thiebaut de Schotten et al., 2012) note the similarity of the human FAT with a fiber pathway described in a single macaque studied with autoradiography. This fiber pathway, reported in Case 25 of Schmahmann and Pandya (2006) is similar to, though

Functional associations of the left frontal aslant tract in speech and language

Given its connectivity with the IFG, which has traditionally been referred to as “Broca's area”, a region important for speech and language (Tremblay & Dick, 2016), and with the pre-SMA and SMA associated with aphasia of the supplementary motor area (Ardila & Lopez, 1984) and with speech production in typical people (Tremblay & Gracco, 2009), it is not surprising that the vast majority of studies on the function of the left FAT have focused on speech and language. Indeed, there is extensive

Functional associations of the right frontal aslant tract in executive function/inhibitory control

Although speech and language functions are distributed across several regions on both hemispheres, some aspects of speech and language are left lateralized in most right-handed individuals (Knecht et al., 2000), and the function of the left IFG has been a focus of inquiry since the time of Broca. The functional association of the right IFG has only more recently become a subject of debate. Earlier reports focused on the role of the right IFG in executive function, specifically inhibitory

Proposed function of the frontal aslant tract

As Schwan and colleagues (2012) point out, the limited physiologic data on this particular connection in humans makes it difficult to specifically determine its function at a more mechanistic level. Yet, several groups have hinted at proposed functions of the FAT, especially for its general involvement in speech. For example, Catani and Bambini (2014) proposed a broad role for the FAT in providing “a basis for intentional communicative acts”, with a potential role in social cognition. However,

Limitations and suggested areas of future research

The proposed model suggests a number of potential avenues for future research. We will focus on a few here. First, with respect to the left FAT, although we have argued for an important but perhaps not primary role for the FAT's in sequential movement planning for speech, we have grounded this on a limited empirical base investigating the tract's specific functions. Very limited research has examined different sub-components of the FAT and their associated functions. Thus, it may be the case

Conclusion

The available data suggest the existence of direct pre-SMA/SMA and IFG connectivity. We propose that this connection is a key pathway for two important functional circuits—speech and executive function/inhibitory control—that are typically examined separately but that rely on overlapping mechanisms. What this review shows is that cross-pollination of the models of these circuits may be beneficial for understanding each of them separately. In addition to understanding the basic circuitry of

Conflicts of interest

None.

Acknowledgements

This work was supported by grants from the National Institutes of Mental Health (Grant R01MH112588 and R56MH108616 to P.G. and A.S.D), and from the National Institute on Drug Abuse (U01DA041156; salary support to A.S.D.). P.T is supported by a career award from the from the “Fonds de la Recherche en Santé du Québec” (FRQS, #35016).

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